ok guys, got my blackbear tune a few months back, now I've got a code showing drivers side cat is bad (didn't ask my buddy the code #- he told me that's what it is). not wanting any louder than i have now with my magnaflow setup, and not looking for high $$ performance gains- just a replacement. are these thunderbolt cats i've seen online good? they seem pretty inexpensive. which size for 99 5.3?

 

first of all.
swap the sensors and scan it again. if the code changes places its your sensor and not the cat. just so you don't buy what you don't need.

 

i'll try that, but anybody else got any ideas about replacement cats? wouldn't it show an O2 sensor code if that was the problem?

 

I think that was a good suggestion and should take all of 5 minutes to do. Magnaflow cats are good, don't know about the brand you mentioned.